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A series of siloxane based side chain liquid crystal polymers have been prepared with asystematic variation in spacer length. Nematic liquid crystal polymers possess large optical nonlinearities owing to their large refractive index anisotropy coupled with the collective molecular reorientation. All the polymer exhibited a smectic phase, for which the Nematic -isotropic transition temperature increased as the spacer length increased. Electro-optic measurements are used to evaluate the threshold voltages for this series of polymers. It is found that with increasing spacer length (n) of polymer the threshold voltage is lowered and that the variation of the threshold voltage arises from changes to the intrinsic curvature elasticity rather than to differences in orientational order. A simple model is used to indicate the origion of the effects observed which appear to arise from the constraints offered by the coupling of the mesogenic units to the polymer backbone.